1. Field of the Invention
The present invention refers to a current lead for superconducting electrical apparatus.
2. Description of the Related Art
The term “superconducting apparatus” denotes an electrical device such as cable or magnet operating in conditions of so-called superconductivity, i.e. in conditions of almost null resistivity. See, for example, Engelhardt J. S. et al., Application Consideration for HTSC Power Transmission Cable, 5th Annual Conference on Superconductivity and Application, Buffalo, N.Y., Sep. 24-26, 1991, and Yukikazu Iwasa, “Case Studies in Superconducting Magnets”, New York, Plenum Press, 1994.
A superconducting cable typically comprises at least one phase conductor including superconducting material, at least one cryogenic fluid flowing channel, an electric insulator (dielectric) and a cryostat generally including two coaxial tubes, and between them a gap, generally under vacuum or at a very low pressure, for example of 10−5-10−6 mbar, or containing a low thermal conductive gas, at least partially filled with a thermal insulator. The cryogenic fluid flowing in said at least one channel is typically helium, nitrogen, hydrogen, argon or mixture thereof, at the liquid or gaseous state, and operates at temperature and pressure specific for the application.
The term “superconducting material” indicates a material having a superconductive phase with substantially null resistivity at temperature values equal or lower a threshold value, defined as critical temperature (Tc). For example, special ceramics based on mixed oxide of copper, barium and yttrium (generally known as YBCO), or of bismuth, lead, strontium, calcium and copper (generally known as BSCCO) have Tc ranging from about 60 K (−213° C.) to about 170 K (−103° C.), being commonly denoted with the name of high temperature superconducting (HTS) material.
Advantageously, the operative temperature of a superconducting apparatus is lower than the critical temperature of the superconducting material contained therein, so as to guarantee a safety margin in case of disfunctioning of the devices for setting and maintaining the proper thermal conditions. These devices mainly include said cryogenic fluid flowing into at least one channel, and said cryostat.
Conveniently, each component of said cable should guarantee a minimal heat transmission to the cryogenic fluid, so that the cooling investment and maintenance costs are decreased.
As, for example, said in U.S. Pat. No. 5,991,647, a major source of heat escaping into a cryogenic system typically occurs at the connection of the cryogenic system to the outside world. When the cryogenic system houses an operative superconducting device immersed in a cryogenic fluid, the “connection” is typically through the current leads, through which electrical energy passes to and from the device. The escape of heat through this connection is often referred to as “heat leak”.
Typically, a current lead is composed by    a) at least a conductor, where the current runs through,    b) a coolant system generally comprising a cryogenic fluid, such as helium or nitrogen, and pipes for exhausting the vapors of such fluid,    c) a thermal insulation,    d) means for electrically connecting said conductor to both the superconducting termination and the power generation/distribution network (briefly in the following “electric network”), typically operating at temperatures higher than the operative temperature of the superconducting apparatus.
U.S. Pat. No. 4,695,675 discloses an electric lead device for a superconducting electric apparatus, in which the total cross sectional area of conductors in the lead housing can be reduced from the normal temperature side toward the cryogenic temperature side, by stepwise decreasing the number of conductors from the normal temperature side toward the cryogenic temperature side. The conductors are cooled by introducing coolant gas vaporized from a cryogenic coolant, such as helium, stored in a tank. The Applicant observes that the current lead is physically separated from the superconducting apparatus.
U.S. Pat. No. 4,754,249 discloses a current lead structure for superconducting electrical apparatus wherein the conductor(s) is spirally configured. The applicant observes that the term “spirally” is incorrectly used therein, as FIG. 2 of this patent is showing a conductor in helical form, not in a planar configuration as would be required according to the geometric meaning of the word “spiral”. Such “helical” configuration greatly increases the overall length of the conductor and thus the length of the heat conduction path defined thereby, while at the same time correspondingly increasing its overall surface area to thus enhance the cooling of the conductor by the exhausting helium gas. At the same time, this configuration of the conductor enables its axial length to be reduced as necessary to accommodate design size criteria for the overall apparatus. This current lead is physically separated from the superconducting apparatus. The conductor is disposed in a pipe having thermal and electrical insulating material applied to its inner surface. Also, as cooling medium acting on the conductor surface, the gaseous phase of a liquid coolant and pipe(s) for exhausting said gaseous phase are used.
The use of a superconducting conductor in the current lead has been proposed. U.S. Pat. No. 5,880,068 relates to a high-temperature superconductor (HTS) lead for a cryogenic magnetic system, formed by a plurality of HTS lead elements including a series of HTS plates disposed each other in non-collinear arrangements or “zig-zag” configuration. Said HTS lead plates are composed by stacks of superconducting tapes and are placed onto a support with ends connected to copper end pieces. The cryogenic magnet system includes an enclosure, for example, a dewar cylinder held under a vacuum, containing a low or high temperature superconductor magnet and high-temperature superconductor leads.
The heat generation has to be controlled in view of the near superconducting apparatus requiring the above-mentioned thermal conditions (very cold environment) to operate. Such a control is effected by a refrigeration system generally comprising a cryogenic fluid, such as helium or nitrogen, contacting the surface of the conductor. The heat leak causes the cryogenic fluid to warm up. Due to the temperature difference and limitations of liquefier efficiency, the refrigeration power required to recondense the cryogenic vapor back to the liquid state or to cool down the cryogenic fluid generally is several hundred to over a thousand times the heat leak to the cryogen pool. A substantial reduction in refrigeration system capital cost as well as the operating cost can therefore be achieved by the reduction of heat leak to the cryogenic system.
The heat generation due to Joule effect can be reduced by increasing the conductor section (S), thus decreasing the resistance, while the heat conduction can be reduced by increasing the length (L). Yukikazu Iwasa, supra, page 137 (eq. 4.25), discloses that for minimizing the heat leak to the cooling medium, the ratio L/S has to be
      L    S    =                              2          ⁢                                          ⁢                                    k              ~                        ⁡                          (                                                T                  1                                -                                  T                  0                                            )                                                ρ          ~                      ·          1              I        t            
wherein
L is the length of the current lead conductor(s) from the room (or normal) temperature end to the cryogenic temperature end, i.e. the length over which the temperature gradient occurs;
S is the conductor section;
{tilde over (k)} is the average thermal conductivity
{tilde over (ρ)} is the average electrical resistivity
T1 is the higher temperature end
T0 is the lower temperature end, and
It is the current flowing through the current lead.
Thus, for minimizing the heat leak to a cooling medium, an increase of the section S has to correspond to an increase of the length L. It implies an overall size increasing of the current lead for minimizing both the heat leak and the refrigeration system cost, but this size increasing is not desirable in view of constructive and practical demands.
When superconducting material for the current lead conductor is employed, there is anyway an interface between the power generation/distribution network operating at higher temperature and the superconducting apparatus, hence the above mention problems of heat leak show up, before or after.
Applicant perceived that a current lead conductor with an extended length for decreasing the heat conduction and a reduced irradiating surface for restricting or even avoiding the use of a cryogenic fluid in direct contact with the conductor, could reduce the refrigerating system cost. An overall size reduction of the current lead conductor is also perceived as profitable.